U.S. Pat. Nos. 3,560,641, 3,586,907 and 3,661,304 are directed to noncontacting coating systems wherein a liquid coating material, such as ink, is pumped under pressure to a manifold communicating with a series of small diameter orifices. As the coating material is ejected through the orifices under pressure, it forms fine filaments of coating material which break down into series of discrete drops. At the point where the drops break from the filaments they pass through charging rings which, depending upon the pattern of coating material desired on a receiving member conveyed beneath the drop generator, either charge or do not charge each individual drop of coating material.
An electrostatic deflecting field is set up downstream of the charge rings and all drops which receive a charge from the charge rings are deflected from their trajectory by the deflecting field. A catcher is also associated with the system to catch those drops which it is desired to prevent from reaching the receiving member. In this way it is seen, a pattern coating, such as printing, is applied to the receiving member.
In the operation of a drop generator of this type, it will be apparent that it takes some discrete pressure, hereinafter termed the operating pressure, to produce a filament of sufficient velocity to overcome forces, such as surface tension forces, tending to retard flow of the coating material through the orifices.
If the flow of coating material to the drop generator is commenced by merely opening a supply line to the manifold, it will be apparent that the pressure build up in the drop generator from zero to the operating pressure will occur over a finite time period.
During this period, when the pressure acting on the coating material has not yet reached operating pressure, a free jet will not be produced, but instead, a pendulous mass of coating material will collect at each orifice which weeps liquid coating material therefrom. As the pressure acting on the coating material increases a jet will eventually be produced inside the mass of liquid and finally break from the mass in an uncontrolled manner, only stabilizing after the excess liquid at the orifice has been drawn away by entrainment in the jet.
Obviously this will result, not only in a more lengthy start up procedure, but also in spattering of the coating and the collection of coating material on the components of the generator. Since the coating material is electrically conductive this can result in shorting of the various electrical components, such as the charge rings and deflecting field electrodes. Additionally, the evaporation of the coating material will leave a residue on the components of the drop generator which will eventually affect its operation.